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# Bi-directional tuning of thermal transport in SrCoOx with electrochemically induced phase transitions

## Abstract

Unlike the wide-ranging dynamic control of electrical conductivity, there does not exist an analogous ability to tune thermal conductivity by means of electric potential. The traditional picture assumes that atoms inserted into a material’s lattice act purely as a source of scattering for thermal carriers, which can only reduce thermal conductivity. In contrast, here we show that the electrochemical control of oxygen and proton concentration in an oxide provides a new ability to bi-directionally control thermal conductivity. On electrochemically oxygenating the brownmillerite SrCoO2.5 to the perovskite SrCoO3–δ, the thermal conductivity increases by a factor of 2.5, whereas protonating it to form hydrogenated SrCoO2.5 effectively reduces the thermal conductivity by a factor of four. This bi-directional tuning of thermal conductivity across a nearly 10 ± 4-fold range at room temperature is achieved by using ionic liquid gating to trigger the ‘tri-state’ phase transitions in a single device. We elucidated the effects of these anionic and cationic species, and the resultant changes in lattice constants and lattice symmetry on thermal conductivity by combining chemical and structural information from X-ray absorption spectroscopy with thermoreflectance thermal conductivity measurements and ab initio calculations. This ability to control multiple ion types, multiple phase transitions and electronic conductivity that spans metallic through to insulating behaviour in oxides by electrical means provides a new framework for tuning thermal transport over a wide range.

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## Data availability

Experimental and computational data are available from the corresponding authors on reasonable request.

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## Acknowledgements

This work was supported primarily by the MRSEC Program of the National Science Foundation under award number DMR-1419807. This work made use of the Shared Experimental Facilities supported in part by the MRSEC Program of the National Science Foundation under award number DMR-1419807. This research used the IOS Beamline of the National Synchrotron Light Source II, a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under contract no. DE-SC0012704. We thank P. Yu from Tsinghua University and his co-workers for sharing the crystal structure files of H-SCO used for the visualization in Fig. 1a. The authors acknowledge the support of computational resources from the National Energy Research Scientific Computing Center (NERSC), a US DOE Office of Science User Facility operated under contract no. DE-AC02-05CH11231; the MIT-PSFC partition of the Engaging cluster at the MGHPCC facility, which was funded by DOE grant no. DE-FG02-91-ER54109, and the MIT-NSE partition funded by MIT; the Extreme Science and Engineering Discovery Environment (XSEDE) Stampede2 at Texas Advanced Computing Center through allocation TG-DMR120025, which is supported by National Science Foundation grant no. ACI-1548562. Q.S. thanks B. Song, K. Chen and J. Zhou for help with TDTR measurements. H.Z. thanks O. Hellman, T. Tadano, J. Yang, L. Sun and Y. Chi for technical help and fruitful discussions.

## Author information

Authors

### Contributions

Q.L. and J.W. prepared the SCO thin-film samples and performed the electrochemical gating of SCO films by using YSZ, ionic liquid and ion gel as the electrolytes. S.H. and Q.S. performed the TDTR measurements, and H.Z. did the first-principles simulations. Q.L., G.V., I.W. and A.H. performed the XAS measurements. All the authors discussed the results and contributed to the writing of the manuscript. B.Y. and G.C. originated and supervised the research.

### Corresponding authors

Correspondence to Gang Chen or Bilge Yildiz.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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## Supplementary information

### Supplementary Information

Supplementary Figs. 1–16, Discussion andy Tables 1–3.

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Lu, Q., Huberman, S., Zhang, H. et al. Bi-directional tuning of thermal transport in SrCoOx with electrochemically induced phase transitions. Nat. Mater. 19, 655–662 (2020). https://doi.org/10.1038/s41563-020-0612-0

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• DOI: https://doi.org/10.1038/s41563-020-0612-0

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